Method of managing liquid steams in a pulp mill

ABSTRACT

In minimizing effluents from a cellulose pulp mill, liquid effluents from a bleach plant are concentrated, and then incinerated to produce a residue including sodium, sulfate, and sodium chloride. This residue is distilled with sulfuric acid to produce gaseous hydrogen chloride and remaining residue, the HCl being used in chloride dioxide production for the bleach plant, while the remaining residue is passed to the recovery loop (e.g. recovery boiler). Sulfur containing gases from the non-condensible gas system may be combusted to produce gaseous sulfur dioxide, which is then converted to sulfuric acid, to distill the residue. Where a non-chlorine bleach plant is provided, the liquid effluents may be concentrated in evaporators and then passed directly to the recovery boiler. The liquid streams in the mill are managed by a cascade principle to reserve the cleanest water for only those processes where it is needed while minimizing its use where it is not necessary.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.07/922,334 filed Jul. 30, 1992 (attorney reference 10-765).

BACKGROUND AND SUMMARY OF THE INVENTION

It has long been a desire of those working in the paper pulp art toproduce a pulp mill that does not in any way significantly pollute theenvironment. A number of proposals have been made for such a pulp millin the past, but the desired goal has yet to be achieved. For example, a"closed mill" was constructed at Great Lakes Forest Products, ThunderBay, Ontario, in the 1970s, but it was difficult to run the mill closedfor extended periods of time as a result of corrosion problems in therecovery boiler, and elsewhere, due to chloride buildup. See "Bleachingin the Closed Cycle Mill at Great Lakes Forest Products Ltd." byPattyson et al, Pulp & Paper Canada, Vol. 82, No. 6, pp. 113-122 (1981).In the Great Lakes mill, bleaching plant effluents were introduceddirectly into the chemical recovery loop, as shown schematically in U.S.Pat. No. 4,039,372.

More recently, it has been proposed by HPD and Jaakko Poyry that closingof a pulp mill can be accomplished by evaporating acid effluent and thenreturning the E_(o) bleach plant effluent to the brown stock washers.However that approach has yet to be successful, despite the utilizationof plastic falling film evaporators which allow effective evaporation ofthe bleaching chemicals, and it is believed unlikely that it willultimately be successful because of the buildup of undesired chemicalsdue to the introduction of the flow from the E_(o) stage back to thebrown stock washing stage.

According to the present invention, a method and apparatus are providedwhich utilize only existing technology, so that future development ofsophisticated additional equipment or processes is not necessary, whichessentially can reduce the liquid polluting effluents from a pulp millto zero, provide only a minimum amount of solid waste for disposal (andprovide the high probability that such solid waste can be used in anenvironmentally acceptable manner), and minimize the production ofgaseous NO_(x) and SO_(x) products, so that the only significant gaseouspollutant from the pulp mill is carbon dioxide.

One of the basic aspects of the present invention that makes it possibleto achieve these beneficial results is to treat the bleaching effluentscompletely separately from the chemical recovery loop until theeffluents are in a particularly desirable form, and to then introducethe chemicals in that desirable form into the recovery loop. Anothersignificant aspect of the present invention is the essentially completeoxidation of white liquor produced in the chemical recovery loop, whichis then returned to the bleaching stage so that the proper balancebetween the various chemical treatment sequences is provided. Anothersignificant aspect of the present invention that allows the desiredresults to be achieved are the production on site at the pulp mill,directly from the effluent streams and gaseous waste streams themselves,of essentially all of the sulfur dioxide, sulfuric acid, caustic orcaustic substitute, and (if utilized) chlorine dioxide necessary toeffect treatment of the pulp and recovery of the chemicals. Anotherfactor which minimizes the amount of bleach plant effluents so as tomake a proper treatment thereof practical, is advanced digestingtechniques where delignification can be extended so that thepulp--without significant strength loss--discharged from the digestingstages has a low Kappa No. (e.g. 24 or below) and then the pulp issubjected to oxygen delignification to reduce the Kappa No. stillfurther (e.g. to 14 or below, typically 10 or below) before bleaching iseffected, allowing the production of prime market pulp (e.g. 88-90 ISObrightness).

The ability to produce prime market pulp with minimal adverse affect onthe environment, according to the invention, is a quantum leap forwardin pulping technology, and allows fulfillment of a long felt need toaccomplish this desirable result.

According to one aspect of the present invention, a method of minimizingeffluents from a cellulose pulp mill having a digester, bleach plant,and a recovery boiler and chemical recovery loop, is provided. Themethod comprises the following steps: (a) Concentrating (e.g. byevaporation) liquid effluents from the bleach plant to a concentrationlevel high enough for incineration. (b) Incinerating the concentratedbleach plant effluents to produce a residue containing sodium, sulfate,carbonate, and sodium chloride. (c) Leaching the residue to produce aleachate. And, (d) feeding at least a substantial portion of theleachate to the chemical recovery loop associated with the recoveryboiler.

The method also preferably comprises the further steps of: (e) Removingblack liquor from the digester. (f) Increasing the solids concentrationof the black liquor to a level high enough for incineration. (g)Incinerating the concentrated black liquor in the recovery boiler toproduce a melt. (h) Producing white liquor and/or NaOH from materials inthe recovery loop including the melt and the leachate fed to therecovery loop. (i) Oxidizing at least a part of the white liquor. And,(j) using at least a part of the oxidized white liquor in place ofcaustic in the bleach plant.

The invention also contemplates collecting spills of liquid from thepulp mill, evaporating the collected spills, and adding the concentratedspills to the concentrated bleach plant effluents in order to practicestep (b). The spills are typically clarified before evaporation. Therealso are preferably the further steps of treating water removed from thebleach plant effluents by concentrating them, and then using the treatedwater as wash water in the bleach plant and in other mill processes.

Also there preferably are the further steps of producing substantiallyall caustic (or caustic substitute such as essentially completelyoxidized white liquor) for the bleach plant, sulfuric acid, and sulfurdioxide needed for the plant processes, from process effluents andgaseous streams on site at the pulp mill so that no substantial externalsource of supply thereof need be provided.

Prior to feeding the leachate to the recovery loop, it is preferred thatthe leachate be crystallized and washed. The leachate also typicallyincludes sodium chloride, and leachate containing chloride is used inthe plant to produce substantially all of the chlorine dioxide necessaryfor the bleach plant. All of the metals above monovalent are removedfrom the leachate by washing, and those metals are kept out of therecovery loop and away from the bleach plant.

The bleach plant may have both acid and alkali liquid effluents, inwhich case it is desirable to initially evaporate (or otherwiseconcentrate) those different effluents separately, and then combine themfor a final evaporation (concentration) before incineration. One typicalbleaching sequence for the bleach plant may be DE_(o) PD_(n) D (where_(n) refers to a neutralization stage between the two chlorine dioxidestages), and another typical bleaching sequence is AZE_(o) PZP, althougha wide variety of other bleaching sequences may also be utilized.

The invention also contemplates a method of recovering chemicals frombleach plant liquid effluents resulting from the production of chemicalcellulose pulp by the following steps: (a) Concentrating (e.g.evaporating) the bleach plant liquid effluents to produce a concentratedeffluent. (b) Incinerating the concentrated effluent to produce aresidue. (c) Acting on the residue to recover sodium, sulfate, carbonateand/or sodium chloride. And, (d) using the recovered sodium, NaCl,sulfate and/or carbonate in the production of the chemical cellulosepulp.

The invention also contemplates a method of producing cellulose chemicalpulp in a pulp mill, which requires sulfur dioxide, sulfuric acid, andcaustic, and which has process effluents and gaseous streams, comprisingthe step of producing all of the sulfuric acid, sulfur dioxide, andcaustic (or caustic substitute) necessary to effectively producechemical pulp directly at the pulp mill, from the process effluents andgas streams, so that substantially no additional sulfuric acid, sulfurdioxide, or caustic is necessary from external sources.

According to another aspect of the present invention, apparatus forproducing chemical pulp with a minimum discharge of effluents isprovided. The apparatus comprises: A digester. A chemical recovery loopoperatively connected to the digester, and including a recovery boiler.A bleach plant including at least one liquid effluent line therefrom.Concentrating means (e.g. evaporators) connected to the liquid effluentline from the bleach plant to produce a concentrated effluent. Anincinerator for incinerating the concentrated effluent from theevaporator means, for producing a residue. And, means for recoveringsodium, NaCl, carbonate and/or sulfate from the incinerator residue andfeeding at least some of those recovered materials to the recovery loop.Also, water is recovered from the bleach plant effluents, which is usedelsewhere in the mill.

The evaporator means preferably comprise a plurality of stages ofmetal-plastic laminate, falling film evaporators. Such evaporators areavailable from A. Ahlstrom Corporation of Helsinki, Finland, andAhlstrom Recovery Inc. of Roswell, Ga. under the trademark "Zedivap",and described in co-pending application Ser. No. 07/974,060 filed Nov.12, 1992 (corresponding to Finnish Application 915424 filed Nov. 18,1991, and the disclosure of which is incorporated by reference herein).Although other evaporators, such as desalination evaporators, also arefeasible, the "Zedivap"™ evaporators are particularly advantageous andmake the evaporating process for the bleach plant effluents practical.The evaporator means also may further comprise a concentrator betweenthe stages of metal-plastic laminate evaporators and the incinerator.

According to yet another aspect of the present invention, the followingapparatus is provided: A bleach plant for bleaching cellulose chemicalpulp, and producing liquid effluents during bleaching. Means forconcentrating (e.g. evaporating) the bleach plant liquid effluents toproduce a concentrated effluent. An incinerator for incinerating theconcentrated effluent to produce a residue. Means for acting on theresidue to recover sodium, sulfate, NaCl, and/or carbonate. And, meansfor using the recovered sodium, sulfate, NaCl, and/or carbonate in theproduction of the chemical cellulose pulp being bleached.

The invention also contemplates the following apparatus: Means foracting upon all liquid effluents in the pulp mill so that no liquideffluents are discharged from the pulp mill to the environment. And,means for acting on all gaseous effluents from the pulp mill so that theamount of SO_(x) and No_(x) are minimized, and the only major adversegaseous effluent is carbon dioxide.

According to still another aspect of the present invention there isprovided the method of: Digesting comminuted cellulosic fibrous materialto a Kappa No. of about 24 or below. Effecting oxygen delignification ofthe digested pulp to a Kappa No. of about 14 or below. Bleaching theoxygen delignified pulp to produce bleach liquid effluents.Concentrating (e.g. evaporating) the liquid bleach effluents into aconcentrated effluent. Incinerating the concentrated effluent to producea residue. And, acting on the residue to recover chemicals therefromused in the digesting, oxygen delignification, and/or bleaching stages,while also recovering water.

As alternative to the procedures described above, in mills where achemical plant for producing chlorine dioxide is provided for the bleachplant, the method of minimizing effluents may entail the followingsteps: (a) Concentrating liquid effluents from the bleach plant to aconcentration level high enough for incineration. (b) Incinerating theconcentrated bleach plant effluents to produce a residue containingsodium, sulfate, sodium chloride, and carbonate. (c) Distilling theresidue from step (b) with sulfuric acid to produce gaseous hydrogenchloride, and remaining residue. (d) Using the gaseous hydrogen chloridefrom step (c) in the chemical plant for producing chlorine dioxide. And,(e) passing the remaining residue from step (c) to the recovery loop.

Preferably the recovery loop includes a recovery boiler, and step (e) ispracticed to pass the remaining residue either directly to the recoveryboiler, or first to evaporators and then to the recovery boiler. Theremaining residue also may be treated to remove heavy and transitionmetals prior to the practice of step (e). Also the mill typically has anon-condensable gas system, including hydrogen sulfide and methylmercaptan gases therein, and there are further steps of: (f) combustingthe non-condensible gases in the non-condensible gas system to producegaseous sulfur dioxide; (g) converting the gaseous sulfur dioxide tosulfuric acid; and (h) using the sulfuric acid from step (g) in step(c). Step (g) is preferably practiced by the Wet Sulfuric Acid (WSA)process. There may also be the further step (i), between steps (f) and(g), of reacting the gaseous sulfur dioxide from step (f) with sodiumsulfite and water to produce concentrated sodium bisulfite and heatingthe concentrated sodium bisulfite in the presence of a catalyst toregenerate a concentrated form of gaseous sulfur dioxide; and whereinthe concentrated form of gaseous sulfur dioxide from step (i) is used instep (g). Step (i) is known as the RESOX process.

Step (a) is typically practiced by evaporating the liquid effluents fromthe bleach plant, utilizing a plurality of stages of metal-plasticlaminate falling film evaporators as described above.

Where the cellulose pulp mill bleach plant is a non-chlorine plant, amethod of minimizing effluents may comprise the following steps: (a)Concentrating liquid effluents from the non-chlorine bleach plant,having little or no chlorine therein, to a concentration level highenough for combustion in the recovery boiler. (b) Passing theconcentrated non-chlorine bleach plant effluents directly to therecovery boiler. And, (c) combusting the concentrated non-chlorinebleach plant effluents in the recovery boiler.

In all the procedures described, it is highly desirable to properlymanage the water (liquid streams), so that the cleanest liquid isreserved only for those processes where it is needed, and so that itsuse is minimized where the absolutely cleanest water is not needed.Typically the pulp mill has at least first, second, and third loops ofliquid streams, the loops having significantly different contaminationlevels, the contamination level gradually increasing from the first loopto the third loop. Minimizing the use of least contaminated liquid isachieved by: (a) sensing the contamination level in at least the firstand second loops; (b) when the contamination level in the first loopexceeds a predetermined level, discharging some of the contaminatedliquid from the first loop to the second loop, and replacing it withless contaminated liquid; (c) when the contamination level in the secondloop exceeds a predetermined level, discharging some of the contaminatedliquid from the second loop to the third loop, and replacing it withless contaminated liquid; and (d) purifying the most contaminated liquidfrom the loops to produce the less contaminated liquid for addition toat least step (b).

The method of managing liquid streams as described above is not limitedto the methods heretofore described, but may be practiced in any pulpmill. Typically the method is practiced by combining the mostcontaminated liquid with the liquid effluents from the bleach plant toproduce a combined effluent stream, and effecting evaporation of thecombined effluent stream to produce a clean liquid and a moreconcentrated contaminant stream, with the clean liquid used as the lesscontaminated liquid in the steps described, while the more concentratedcontaminant stream is further treated to recover contaminants therefrom(e.g. by incinerating and recovering chemicals from the incinerationresidue, etc., as earlier described). The pulp mill also typicallyincludes a fourth loop and the contamination level in the third loop issensed and when it exceeds a predetermined level some of thecontaminated liquid is discharged from the third loop to the fourthloop, and replaced with less contaminated liquid. Also the contaminationlevel in the fourth loop is preferably sensed, and when it exceeds apredetermined level liquid is discharged from it for the purificationstep earlier described. Liquid spills that are recovered from the pulpmill may be introduced into the fourth loop. There is also preferablythe step of cooling the liquid in the loops, and in at least some of theloops separating contaminants from the loops and passing those separatedcontaminants to a more contaminated loop. Some of the contaminatedliquid from the first loop may be passed directly to the third loop.

The invention also contemplates apparatus for managing liquid streams ina cellulose pulp mill having a digester, a bleach plant, and at leastfirst, second, and third loops of liquid streams. The loops havingliquid streams therein of significantly different contamination levels,the contamination level gradually increasing from the first loop to thethird loop. This water managing apparatus comprises means for sensingthe contamination level in at least the first and second loops; valvemeans controlled by the sensing means for discharging some of thecontaminated liquid from one loop to another, more contaminated, loop;and means for purifying contaminated liquid (typically the mostcontaminated) from the loops to produce less contaminated liquid. Theapparatus may also include means for cooling the liquid in each of theloops. Also, at least one of the loops includes means for separating arejects stream from the loop, and for discharging the reject stream toanother, more contaminated, loop

It is the primary object of the present invention to provide for theproduction of cellulose chemical pulp with essentially zero discharge ofliquid pollutants to the environment, with a minimum amount of gaseouspollution, and with the minimum amount of solid waste products. This andother objects of the invention will become clear from an inspection ofthe detailed description of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the most basic components of one exemplarysystem according to the present invention, and for practicing exemplarymethods according to the present invention;

FIG. 2A and 2B are a flow sheets similar to that of FIG. 1, only showinga number of the particular processes involved in more detail;

FIGS. 3A and 3B are schematics of an alternative system according to thepresent invention based upon the same concepts as the systems of FIGS.1, 2A and 2B only showing different details of the handling of bleachplant effluents, the particular bleach plant stages involved, and thelike;

FIG. 4 is a view like that of FIGS. 2A and 2B, only simplified, andshowing alternative exemplary treatment of concentrated liquid effluentsfrom the bleach plant, according to another aspect of the presentinvention;

FIG. 5 is a schematic view like that of FIG. 4 only showing yet anothermodification of treatment of the bleach plant effluents, when the bleachplant is a chlorine-free bleach plant;

FIG. 6 is a schematic view of a portion of the system of FIG. 2A onlyshowing modified treatment of gases, including for the production ofsulfuric acid; and

FIG. 7 is a schematic view showing exemplary apparatus according to theinvention for practicing an exemplary method according to the inventionfor managing water (liquid) streams in a pulp mill to reserve thecleanest liquid for only those processes where it is needed, whileminimizing its use where it is not needed, and thus optimizing thechances of achieving the goal of "zero discharge".

DETAILED DESCRIPTION OF THE DRAWINGS

The exemplary system illustrated in FIG. 1 includes a conventionaldigester 10, such as a Kamyr® continuous digester, to which hard wood orsoft wood chips, or other comminuted cellulosic material, is fed. In thedigester 10 the wood chips are acted upon by the cooking chemicals atconventional temperature and pressure conditions so as to producechemical cellulose pulp, such as kraft pulp, which then is preferablysubjected to oxygen delignification at stage 11. According to thepresent invention it is desirable to delignify the pulp so that it has aminimum Kappa No. when discharged from the digester 10, such as by usinga Kamyr EMCC® digester and process, which produces a Kappa No. of about24 or below. The oxygen delignification stage 11 reduces the Kappa No.to about 14 or below, preferably to about 10 or below.

After oxygen delignification, the pulp proceeds to the bleach plant 12where it is subjected to bleaching in a plurality of different bleachingstages. The particular bleaching stages that are utilized can be varied,and are also dependent upon the particular cellulose material beingtreated. After the bleaching stages 12, the pulp may proceed on tostorage or further treatment stages 13. For example the pulp may bedried and then shipped to a paper mill.

As is conventional, black liquor is withdrawn from the digester 10 (orbrown stock washer associated therewith), and is passed to evaporators14. The black liquor also is preferably subjected to heat treatment suchas shown in U.S. Pat. No. 4,929,307 (the disclosure of which is herebyincorporated by reference herein). Sulfur containing gases driven off bythe heat treatment 15 may be handled as shown in co-pending applicationSer. No. 07/788,151 filed Nov. 5, 1991 now abandoned, for example toproduce high sulfidity liquor at stage 16, where the production of fuelgas (e.g. primarily methane) as indicated schematically at 17, makespossible generation of power as indicated generally at 18.

After treatment at stage 15 the black liquor is ultimately passed (theremay be intervening evaporation stages if desired) to a conventionalrecovery boiler 19. Steam produced from the recovery boiler 19, asindicated generally at 20 in FIG. 1, is used for various processeswithin the pulp mill. The gases discharged from the recovery boiler 19include sulfur dioxide which can be used as the feed material for theproduction of sulfuric acid according to conventional techniques. Asindicated at 21 in FIG. 1, sulfur dioxide and sulfuric acid (producedfrom the SO₂) can be used wherever necessary in the mill. For examplethe sulfur dioxide is used as an anti-chlor for the last stage ofchlorine dioxide bleaching (if utilized), and for the tall oil plant.According to the invention, sufficient sulfur dioxide and sulfuric acidare available from block 21 to fulfill the needs of the pulp millwithout requiring those chemicals from an external source. While ofcourse one cannot expect the chemical recoveries and consumptions tobalance exactly, according to the invention they may be expected to bewithin a few percent of each other. Of course any small amount of excesschemical can be sold, and any deficiency made up by purchase.

The melt from the recovery boiler 19, as is conventional, is used toform green liquor as indicated by reference numeral 22 in FIG. 1, andthe green liquor is then preferably ultimately used to make whiteliquor, as indicated generally by reference numeral 23 in FIG. 1.Alternatively, or in addition, the green liquor may be crystallized andotherwise acted upon to produce essentially sulfur free sodiumhydroxide, as disclosed in co-pending application Ser. No. 07/918,855filed Jul. 27, 1992 (attorney docket 30-199), the disclosure of which isincorporated by reference herein.

The sulfur content of the melt may be adjusted by bringing a portion ofthe melt discharged from the recovery boiler 19 into contact with asulphurous gas of the pulp mill. Also, one can thermally split themethyl mercaptan and dimethyl sulphide of the sulphurous gas into etheneand hydrogen sulphide before it is brought into contact with the melt,or into contact with ash from the recovery boiler 19. Any white liquorproduced from this melt will have controlled and/or enhanced sulfidity.These techniques are disclosed in Finnish Applications 914585 and914586, both filed Sep. 27, 1991.

Some of the white liquor is fed via line 24 back to the digester 10, andaccording to the present invention, in order to balance the chemicalflows, it is highly desirable that a portion of the white liquor from 23be oxidized at stage 25 in a conventional or known manner, and then usedin the oxygen delignification stage 11. One known manner of oxidationtermed "bubbleless membrane aeration" is described in an article byMichael Semmens in the April, 1991 edition of "WATER/Engineering &Management", pp 18 & 19. Also, a portion of the oxidized white liquorfrom 25 is preferably subjected to a second oxidation stage 26--such asshown in co-pending application Ser. No. 07/910,874, filed Jul. 9, 1992now abandoned (the disclosure of which is hereby incorporated byreference herein)--in order to oxidize all of the sulfur forms withinthe white liquor to sulfates. The resulting essentially completelyoxidized white liquor is then returned to the bleaching plant 12 andused in place of caustic in the bleach plant 12. Sufficient oxidizedwhite liquor can be produced in 26 according to the invention so thatall of the caustic needs for the bleach plant 12 are taken care of,without the necessity of requiring caustic from an external source.

Also according to the present invention, the liquid effluents from thebleach plant 12--such as the acid effluent in line 27 from the firstbleaching stage, and the alkali effluent in line 28 from the secondbleaching stage--are concentrated, e.g. by passage to evaporator stages29, 30, respectively. The evaporators which comprise the stages 29, 30preferably are low cost metal-plastic laminate, falling filmevaporators, such as sold by A. Ahlstrom Corporation of Helsinki,Finland and Ahlstrom Recovery Inc. under the trademark "Zedivap". Suchlaminates are typically of aluminum (or brass or copper) and plastic(e.g. polyethylene, polypropylene, or polyester), each layer having athickness of less than 100 μm. For example an aluminum layer may be 9-18μm thick, and a polyester layer 12-25 μm thick. A plastic film may beextruded on a metal foil to produce a laminate. A heat exchanger isformed by attaching two rectangular laminated strips to each other, forexample by a glued joint. The laminated strips may also be connected toeach other by dot-like junction points between the joints at the edges.The pulp mill liquids may flow down the plastic layer, or the metallayer. Such an evaporator surface is disclosed in co-pending applicationSer. No. 07/974,060 filed Nov. 12, 1992 (corresponding to FinnishApplication 915424 filed Nov. 18, 1991, and the disclosure of which isincorporated by reference herein). However, conventional desalinationevaporators may be used instead.

Where both acid and alkali liquid effluent lines 27, 28 are provided, itis desirable not to mix them until the effluents have been concentratedin the evaporators 29, 30 otherwise a severe foaming problem may ensue.If the foaming problem can be overcome, then the lines 27, 28 may becombined before the evaporators 29, 30.

After the stages 29, 30, the more concentrated effluent passes to theconcentrator 31, which comprises a series of high-efficiency evaporatorstages which concentrate the effluent to a sufficient level so that itcan be incinerated. For example, the concentration of the effluent inlines 27 and 28 may be 0.2-0.5% solids, which is concentrated to asolids content of about 10-30% by the evaporators 29, 30, and thenconcentrated to a concentration of about 50-60% by the concentrator 31.

Concentration of the bleach plant effluents may be accomplished by othertechniques aside from evaporation. For example, conventionalultra-filtration, reverse osmosis, freeze crystallization, or acombination of these techniques with each other and/or with evaporation,may be utilized to produce effluent with a sufficiently highconcentration.

The concentrated effluent from the concentrator 31 or the like is fed toan incinerator 32 where it is burned to produce a residue. Incinerationmay be practiced according to a number of conventional or knowntechniques, such as slagging combustion or gasification (as by means ofa circulating fluidized bed gasifier).

Valuable chemical components of the residue from incinerator 32 areultimately returned to the recovery loop (i.e. components 14, 15, 19,22, 23, etc.). In order to effectively return valuable components of theresidue, such as sodium, sulfate, and carbonate, the residue ispreferably leached by a conventional leaching apparatus, as indicated at33 in FIG. 1. Preferably, the leachate from the leaching stage 33 iscrystallized (e.g. freeze crystallized; see U.S. Pat. Nos. 4,420,318,4,505,728, and 4,654,064) and washed as indicated at 34. Leaching andcrystallizing per se (although in a recovery loop) are known asindicated by TAPPI Journal Volume 66, No. 7, July, 1983 "RecoveringChemicals in a Closed Sulfite Mill" by Davies et al.

The crystallized and washed leachate from stage 34 (or at least aportion thereof) is fed--via line 35--to the recovery loop, such as justbefore the recovery boiler 19. In that way the valuable chemicals fromthe bleach plant effluent in lines 27, 28 are returned to the recoveryloop. The washing separates out metals above monovalent, such as calciumand magnesium, which may be land-filled or treated--as indicated at 36in FIG. 1. The solid material at 36 is essentially the only solid wastematerial from the pulp mill of FIG. 1, and only comprises about 5% ofthe chemicals from the residue of incinerator 32, the other 95% beingused elsewhere (e.g. in the recovery loop).

The residue from the incinerator 32 also typically includes sodiumchloride, and the chlorine content thereof can be used--as indicated bydotted line 37 and box 38 in FIG. 1--to produce chlorine dioxide andsodium chloride. In this circumstance, some of the leachate from stage34 flows to the chlorine dioxide production stage 38, while the rest isreturned to the recovery loop via line 35.

In many pulp mills, regardless of age, the amount of spill liquid can bea significant percentage of the total liquid effluents. Spill liquids ashigh as 33% of a mill total liquid effluents (including the bleach plantliquid effluents in lines 27, 28) are not unusual. Of course if suchspills are allowed to leak into the environment, then the goal of a lowor zero discharge mill will not be realized. Therefore according to thepresent invention, the liquid spills--preferably from the entire pulpmill--are collected utilizing conventional drainage and collectionsystems, as indicated schematically at 39 in FIG. 1. Those spills arethen clarified in the clarifier 40, and passed to spill storage 40' andthen to the evaporator stages 41. The evaporators in stages 41 arepreferably Zedivap™ evaporators. The concentrated spills from theevaporators 41 are then combined with the concentrated effluents fromevaporators 29 and 30, and passed to concentrator 31.

Of course all of the evaporator stages 29, 30, and 41 will producewater, which has been removed from the bleach plants effluents duringthe concentrating action thereof. The water from each of the evaporatorstages 29, 30, and 41 is passed to a water treatment facility 42 whichtreats it so that it does not have any components which are harmful ifthe water is used for other purposes. This "recovery" of water is also abig advantage of the method and apparatus according to the invention.Part of the water is then returned, via line 43, to the bleach plant 12to serve as wash liquid flowing countercurrently to the pulp from onestage to another in the bleach plant 12, while another part of the waterpasses in line 44, which goes to the recovery boiler 19 as feed water,for the production of process steam at 20.

FIG. 2 provides an illustration of the same basic system, for practicingthe same basic method, as in FIG. 1, only shows a number of thecomponents in more detail. In the illustration of FIG. 2 componentscomparable to those in FIG. 1 are shown by the same reference numeral.

In the illustration in FIG. 2, a wood yard 45 is shown connected to thedigester 10, and also to a conventional hog fuel boiler 46. A brownstock washing stage 47 is disclosed after the digester 10, as well as ascreen room 48 cooperating with a press 49, the press 49 also connectedto the clarifier 40. Downstream of the oxygen delignification stage 11is a further washing stage 50, which is then connected to the firststage 51 of the bleach plant 12. In the embodiment illustrated in FIG.2, the first bleaching stage 51 is a 100% chlorine dioxide stage. Thesecond stage 52 is an E_(op) stage, a source of caustic being providedby the oxidized white liquor from 26. A third bleach stage 53 is aneutral chlorine dioxide stage. That is a portion of the oxidized whiteliquor from source 26 (or caustic) is added to the top of the tower ofstage 53 in order to neutralize the pulp acidity. The fourth stage 54 isa last chlorine dioxide stage. Chlorine dioxide from the productionstage 38 is fed to each of the stages 51, 53, and 54, while a portion ofthe wash water from the water treatment plant 42 enters the fourth stage54.

The further treatment stages 13 in the FIG. 2 illustration include the"wet end" 55 and dryer 56, which may be connected to a storage facility57'.

As part of the recovery system, other conventional components areillustrated in FIG. 2, such as the green liquor clarifier 57, the slaker58 for causticizing the green liquor, and the lime mud handlingcomponents including the mud filter 59, pre-coat filter 60, lime kiln61, etc.

Associated with the components acting upon the bleach plant effluents isthe dregs stage 63, which may be supplied with the higher thanmonovalent metals from the crystallizing and wash stage 34, as well asfly ash from the hog fuel boiler 46. The materials from the dreg stage63 may be passed to a land-fill 64, or treated to recover the chemicalstherefrom, or the chemicals therein can be utilized in anenvironmentally acceptable manner.

Also illustrated in FIG. 2 is an optional ozone treatment stage 65 fortreating water from the water treatment plant 42. The water from plant42 is ozonated before flowing to the feed water source 66 which suppliesthe recovery boiler 19, and which also receives water from the dryer 56.Water from the wet end 55 may pass to the water treatment plant 42, orto the interface between the second and third bleaching stages 52, 53.

FIG. 3 illustrates another alternative system according to the presentinvention. One of the major differences between the system of FIG. 3 andthat of FIGS. 1 and 2 is in the particular bleach sequence which isprovided, namely an AZE_(o) PZP bleach sequence. In FIG. 3 componentscomparable to those in the FIGS. 1 and 2 embodiments are shown by thesame reference numeral only preceded by a "1". Also FIG. 3 schematicallyillustrates a number of the components used in the system rather thanmerely showing them in block diagram, as in FIGS. 1 and 2.

The digester 110 may be part of a two vessel hydraulic system, includingan impregnation vessel 68, such as an EMCC® digester sold by Kamyr, Inc.of Glens Falls, N.Y. A pressure diffuser, 69, or similar brown stockwasher may be downstream of the digester 110, which in turn is connectedto high-density storage tank, 147, and then the brown stock screen room148. The oxygen delignification reactors 111 are connected to the postoxygen washing stage 150, which is then connected to the first bleachstage 70, in this case an acid, "A", stage. The second stage of thebleach plant 112 is the first ozone stage 71, and after a wash 72 theE_(o) stage 152 is provided. Following the E_(o) stage 152 is a firstperoxide stage 73, then the second ozone stage 74, and the secondperoxide stage 75, connected up to the high density storage tank 157'.

In the embodiment of FIG. 3, the acid bleach plant effluent line 127 isconnected to the Zedivap™ evaporator stages 129, just like in the FIGS.1 and 2 embodiment, which in turn are connected to the concentrator 131,incinerator 132, leach stage 133, and crystallizing and wash stage 134.However the alkaline effluent line 128 is not connected up toevaporators, but instead is connected up to the recovery loop, typicallyto the green liquor dissolving tank 122. Also a part of the alkalieffluent in line 128 may be used for causticizing, e.g. connected tostage 158; however, much of the alkali effluent would be added to thepost-oxygen washing stage.

The pulp mills of FIGS. 1 through 3, in addition to producingessentially zero liquid effluent discharges, produce little airpollution. Sulfur dioxide and other sulfur compounds are recovered fromthe recovery boilers 19, 119 stacks, and electrostatic precipitators arealso provided in the stacks. Also, the recovery boilers 19, 119 and allthe other components, such as incinerators 32, 132, are operated so asto have minimal NO_(x) discharge. The major gaseous pollutant, then,from the pulp mill will only be carbon dioxide.

FIG. 4 schematically illustrates a system like that of FIGS. 2A and 2B,only in simplified form, and showing an alternative technique forhandling the concentrated liquids from bleach plant effluentevaporation. In the FIG. 4 embodiment components comparable to those inthe FIGS. 2A and 2B embodiment are shown by the same two digit referencenumeral only preceded by a "2". Most of the components will not bedescribed in detail since they have already been described with respectto the FIGS. 1 and 2 embodiment.

In the FIG. 4 embodiment, the bleach plant 212 is one that includes atleast one stage in which chlorine dioxide is used as a bleachingchemical. The effluents from bleach plant 212 pass in line 81 to theevaporators 229, 230, 241 (which may be a single series of metal-plasticlaminate evaporators, with a plurality of stages), and the concentratedeffluents pass in line 82 to the incinerator 232. Incinerating theconcentrated bleach plant effluents produces a residue containingsodium, sulfate, sodium chloride, and carbonate, as well as heavy andtransition metals. The sodium, sulfate, and carbonate are typically inthe form Na₂ SO₄ and Na₂ CO₃. The residue passes in line 83 to saltrecovery stage 84.

In the salt recovery stage 84, the residue is distilled with sulfuricacid (from any source; however one preferred source will be describedwith respect to FIG. 6) to produce gaseous hydrogen chloride (HCl), andremaining residue. The distillation reaction also produces sodiumsulfate as a precipitate when the sodium chlorine reacts with thesulfuric acid.

The gaseous HCl from salt recovery stage 84 is an ideal chemical to beused in the chemical plant 85 to produce chlorine dioxide. Manufactureof chlorine dioxide in chemical plant 85 is as conventional, and theplant 85 is capable of manufacturing sufficient chlorine dioxide tosatisfy the needs of the bleach plant 212 without any significantsupplementation.

The sodium sulfate that precipitates in the salt recovery station 84 iscombined with the remaining residue and is ultimately directed torecovery. Within the salt recovery stage 84 there preferably isconventional treatment to remove heavy and transition metals in the formof metal hydroxides, which metal hydroxides pass to the dregs treatmentstage 263. The sodium sulfate and sodium carbonate in the remainingresidue, after metals removal, then passes to the recovery loop, forexample first to evaporation stage 214 if in slurry form and a higherconcentration is necessary, or directly to the recovery boiler 219.

Another alternative treatment for the bleach plant effluents isillustrated in FIG. 5. In FIG. 5 components comparable to those in FIG.4 are shown by the same reference numeral. In this embodiment the bleachplant (212) is a non-chlorine bleach plant (such as one using Z, P, andE stages), and the liquid effluents discharged therefrom in line81--once concentrated in evaporators 229, 230, 241--may pass in line 86directly to the recovery boiler (219), there being no necessity offurther treatment thereof. There is little or no chlorine in sucheffluents, and thus once they are concentrated enough they may becombusted directly in the recovery boiler 212.

FIG. 6 schematically shows treatment of non-condensible gases which havebeen collected at the pulp mill and which are utilized to produce thesulfuric acid that is used in the salt recovery stage 84 of FIG. 4. InFIG. 6 components comparable to those in the FIG. 2A embodiment areshown by the same reference numeral only preceded by a "2".

In FIG. 6 the NCG stage 88 is a conventional non-condensible gas system,which is utilized to collect hydrogen sulfide, methyl mercaptan,dimethyl sulfide, and other sulfur containing gases which are the sourceof the malodorous gas emissions associated with old style kraft pulpmills. These gases may emanate from the digester, washers, evaporators,recovery furnace, lime kiln 261, or elsewhere.

According to the present invention, one stream of non-condensible gasesfrom stage 88 may be passed directly to lime kiln 261, where they arecombusted to produce off-gases containing sulfur dioxide, which aretreated conventionally (in "PRCIP" in FIG. 6). A second stream may bepassed to the RESOX stage 89. The RESOX stage 89 uses the RESOX processmarketed by A. Ahlstrom Corporation of Helsinki, Finland. In thiscommercial process, the NCG gases (hydrogen sulfide, MM, DMS, etc.)enter a scrubber where the hydrogen sulfide is absorbed into whileliquor in a conventional scrubber or absorption tower. The increasedsulfidity white liquor is used in pulping. The non-absorbed gases arecondensed in a cooling condenser, resulting in removal of the moistureand liquefication of the gases. They are then incinerated to producehydrogen sulfur dioxide, water, and carbon dioxide gases. This gaseousmixture is scrubbed with a caustic soda and sodium sulfite solution in aconventional scrubber or absorption tower in which the sulfur dioxidereacts with the sodium sulfite to produce sodium bisulfite. Thebisulfite and water are then heated in an evaporator to produce sulfurdioxide and water off gases and an aqueous sodium sulfite solution. Thesulfite is recycled back to the caustic stream in the scrubber, and as asource of pulping chemical.

The sulfur dioxide and water off gases from stage 89 may also then passto WSA stage 90, where they are then condensed to yield streams of waterand sulfur dioxide; some of the sulfur dioxide can be made available formill purposes. The rest of the sulfur dioxide and water off gases instage 90 are converted to sulfuric acid (H₂ SO₄), which passes in line91 to salt recovery stage 84.

The WSA stage 90 thus practices the WSA process, a process marketed byHaldor Topsoe A/S of Lyngby, Denmark. This process is described in anarticle entitled "Production of Sulfuric Acid from Sulfurous Off-Gasesby the Topsoe WSA-2 Process" written by Bendixen and Schouby andpresented at "Sulfur 87" in Houston, Tex., 1987 (which article isincorporated by reference herein).

While it is preferred that the WSA stage 90 use the concentrated gaseoussulfur dioxide gas from the RESOX stage 89, if the WSA stage has acombustion unit in which sulfur dioxide is produced, gas in dotted line92 of FIG. 6 passes from NCG unit 88 directly to the WSA stage 90, whenstage 89 is not in use, or eliminated.

An important aspect of being able to maximize the probability of zerodischarge from the mill is to properly manage the water (liquid) streamsthat are present in a pulp mill. If the cleanest water is used only forthose processes where it is needed, while its use is minimized where itis not needed, then the probability of being able to achieve "zerodischarge" is enhanced. FIG. 7 shows a cascading water managementsystem, for practicing a water management method, according to thecascade principle, of the invention which reserves the cleanest waterfor those processes where it is needed while minimizing its use where itis not necessary. Structures in this embodiment comparable to those inthe FIG. 4 embodiment are shown by the same reference numeral.Structures not heretofore illustrated are shown by two digit referencenumerals starting with "94", or with three digit reference numeralsstarting with "3".

In the exemplary embodiment illustrated in FIG. 7, a cooling water loop94, connected to the cooling tower 93 (see FIG. 6 too) is one of themill main loops. The first loop in the mill, 95, is the seal water loop(circuit) 95, which is operatively connected to the second, coolingwater, loop 94, to a third, fairly contaminated, loop 96, and to a--inthis embodiment--fourth "most" contaminated loop 97, which can includethe spills recovery system 239 for the entire mill therein. From thefirst through fourth loops, the water (liquid) within the loop will havegradually increasing, significantly different, contamination levels, thefirst, seal, loop 95 being the least contaminated, and the fourth loop97 being the most contaminated.

Associated with each of the loops 94-97 is a contamination monitoringmeans, typically a conductivity recorder/controller/probe (commonlyknown as a CRC). For example a CRC 98 is associated with the three-wayvalve 99 in the first loop 95. One exemplary form of such a probe isModel 461 available from TBI of Carson City, Nevada (described generallyin U.S. Pat. No. 4,118,663), and it is connected to a conventionalcontroller (computer) to effect control in response to conductivitymeasurements. The CRC 98 monitors the contamination of the liquidflowing in the loop or circuit 95, and when it becomes higher than apredetermined level, it is vented to one or more more contaminatedloops.

The loop 95 typically also includes a separation device 300 which mayseparate dry solids, metals, salts, fibers, or dirt from the watercircuit 95 (or other loop in which it is placed). The device 300 mayemploy ultra-filtration, reverse osmosis, membrane filtration, or fiberfilter techniques, depending upon the mill and the particular locationin the loop 95. The device 300 has an "accept" line 301, and a "reject"line 302, extending therefrom. The line 302 is connected to a vent line304 from the valve 99, to form a main line 303 which--in thiscase--feeds into the third loop 96. The lesser contaminated water fromthe separation device 300 passes in line 301 to the cooler 305 where itstemperature is reduced by heat exchange with the surroundingenvironment, or other cooling source, and then passes to a furtherthree-way valve 306. The valve 306 is also controlled by the CRC 98 sothat if the contamination level is too high in the first loop 95, someof the contaminated liquid can be discharged through valve 306 into line307, to the second, cooling water, loop 94. The discharge through line307 may take place in addition to, or instead of, the discharge fromvalve 99 in line 304.

The cooling water loop 94 typically also includes a cooling heatexchanger 308, as well as a CRC 309 for controlling the three-way valve310, having a discharge conduit 311. The CRC 309 monitors thecontamination level of the water in loop 94, and if it becomes greaterthan desired, valve 310 is controlled to discharge some of thecontaminated liquid through line 311 to the third loop 96.

The third loop 96 includes a CRC 312 associated with a valve 313, thevalve 313 associated with a separation device 314 like the device 300(although it may separate out different "rejects", and use differenttechniques, depending upon the characteristics of the water in loop 96).The device 314 has a contaminants discharge line 315 which connects upto the main line 316, which is also connected to the discharge line 317from the valve 313. The "accepts" output from the separation device 314passes to the cooler 318. The CRC 312 monitors the contamination levelin the loop 96, and if it becomes greater than desired, the valve 313 iscontrolled to discharge some of the contaminated liquid into the line317, which ultimately ends up in the fourth loop 97.

The fourth loop 97 includes the CRC 319 connected to three-way valve320, the valve 320 in turn being connected to a separation device 321like the device 300. The discharge line 322 for contaminants from theseparation device 321 joins with the discharge line 323 from valve 320to form the main line 324 which connects up to the bleach plant 212effluent line 81. The clean output of the separation device 321 isconnected to the cooler 325, and that in turn may be connected to avalve 326 or the like which may dump into the spills recovery system239, connected by line 327 to the inlet to the valve 320.

The line 81 which contains both the bleach plant effluents as well asthe most contaminated liquid (from line 324) from fourth loop 97 may beconnected up to the evaporators 229, 230, 241, which preferably aremetal-plastic laminate multiple stage evaporators. The evaporators 229,etc. produce a concentrated liquid effluent which is passed to anincinerator 232, or for like treatment (as described above with respectto FIGS. 4 and 5), while the clean water (liquid) therefrom passes intoline 330. An optional water treatment plant 242 (shown in dotted line inFIG. 7, and comparable to the structure 242 in FIG. 4 and 42 in FIGS. 1and 2) may be provided to treat the water in line 330 before it isreturned to the bleach plant 212, or otherwise utilized.

For each of the loops 94 through 97, when contaminated liquid is removedit is, of course, necessary to replace at least a portion of the liquid,otherwise the liquid loop would become depleted. For the second loop 94this is typically accomplished by utilizing the discharge through line307 connected to valve 306. The valve 306--as shown schematically inFIG. 7--may be controlled by CRC 309 and/or the CRC 98, to supplysupplemental water to the loop 94.

The supplemental, make-up, water supply to the first loop 95 istypically provided by line 331 which is an off shoot of line 330,containing clean water. Of course for any of the loops 94 through 97,however, all that is necessary is that the contaminated liquid dischargebe replaced by less contaminated liquid, although for the seal loop 95it is desirable to have the clean water from line 330. FIG. 7 also showsbranch lines 332 and 333 supplying liquid to the loops 96, 97,respectively for make-up purposes, although the volume of make-up liquidto these lines will be fairly low since they are provided with liquidfrom the previous lines (i.e. loop 96 is provided with liquid from bothlines 303 and 311, while loop 97 is provided with liquid from both spillcontainment source 239 and line 316). Since the vast majority of make-upliquid for loops 96 and 97 will be provided by the previous loop orloops, if any make-up liquid from the clean line 330 needs to besupplied through the branches 332, 333, it will be a minimal amount.

It will thus be seen that by utilizing the system as illustrated in FIG.7, the cleanest water can be reserved for only where necessary, yetthere is sufficient water flow through all of the liquid circuits withinthe mill.

It will thus be seen that according to the present invention aneffective method and apparatus have been provided for absolutelyminimizing effluents from a cellulose pulp mill. While the invention hasbeen herein shown and described in what is presently conceived to be themost practical and preferred embodiment thereof it will be apparent tothose of ordinary skill in the art that many modifications may be madethereof within the scope of the invention, which scope is to be accordedthe broadest interpretation of the appended claims so as to encompassall equivalent methods and apparatus.

What is claimed is:
 1. A method of managing liquid streams in acellulose pulp mill having a digester, a bleach plant, and at leastfirst, second, and third loops of liquid streams in the pulp mill, theloops having liquid streams therein of significantly differentcontamination levels, the contamination level gradually increasing fromthe first loop to the third loop; comprising the steps of:(a) sensingthe contamination level in at least the first and second loops; (b) whenthe contamination level in the first loop exceeds a first predeterminedlevel, discharging some of the contaminated liquid from the first loopto the second loop, and replacing it with less contaminated liquid; (c)when the contamination level in the second loop exceeds a secondpredetermined level, discharging some of the contaminated liquid fromthe second loop to the third loop, and replacing it with lesscontaminated liquid; and (d) purifying the most contaminated liquid fromthe loops to produce the less contaminated liquid for addition to atleast step (b) and wherein step (d) is practiced by combining the mostcontaminated liquid with the liquid effluents from the bleach plant toproduce a combined effluent stream, and producing a clean liquid used asthe less contaminated liquid in step (b).
 2. A method as recited inclaim 1 wherein said combined effluent stream is evaporated to produce aclean liquid and a more concentrated contaminant stream, said moreconcentrated contaminant stream is further treated to recovercontaminants therefrom.
 3. A method as recited in claim 2 wherein thefurther treatment of the more concentrated contaminant stream includesincinerating it to produce a residue, and recovering chemicals from theresidue.
 4. A method as recited in claim 2 wherein step (d) is furtherpracticed by passing the combined effluent stream through a plurality ofstages of metal-plastic laminate falling film evaporators.
 5. A methodas recited in claim 1 wherein the pulp mill includes a fourth loop,having liquid therein with a greater contamination level than the thirdloop; and wherein step (d) is practiced by using liquid from the fourthloop.
 6. A method as recited in claim 5 comprising the further steps of(e) sensing the contamination level in the third loop; and (f) when thecontamination level in the third loop exceeds a third predeterminedlevel, discharging some of the contaminated liquid from the third loopto the fourth loop, and replacing it with less contaminated liquid.
 7. Amethod as recited in claim 6 comprising the further steps of (g) sensingthe contamination level in the fourth loop; and (h) when thecontamination level in the fourth loop exceeds a fourth predeterminedlevel, discharging liquid from the fourth loop to provide the liquid forthe practice of step (d), and replacing the discharged liquid with lesscontaminated liquid.
 8. A method as recited in claim 7 wherein only step(b) is practiced by using purified liquid from step (d) as the lesscontaminated replacement liquid.
 9. A method as recited in claim 7wherein step (d) is practiced by combining the most contaminated liquidwith the liquid effluents from the bleach plant to produce a combinedeffluent stream, and effecting evaporation of the combined effluentstream to produce clean liquid and a more concentrated contaminantstream, the clean liquid used as the less contaminated liquid in steps(b), (c), (f), and (h) while the more concentrated contaminant stream isfurther treated to recover contaminants therefrom.
 10. A method asrecited in claim 6 comprising the further steps of recovering liquidspills from the pulp mill, and introducing the liquid spills into thefourth loop.
 11. A method as recited in claim 1 comprising the furtherstep of cooling the liquid in at least one of the loops.
 12. A method asrecited in claim 1 comprising the further step (e) of separating arejects stream from at least one of the loops, and passing the separatedreject stream to a more contaminated loop.
 13. A method as recited inclaim 12 wherein step (e) is practiced in at least the first loop, andby passing the separated rejects stream from the first loop to the thirdloop.
 14. A method as recited in claim 13 wherein step (b) is alsopracticed to pass some of the liquid from the first loop directly to thethird loop.
 15. A method as recited in claim 14 wherein the rejectsstream from step (e) separated from the first loop is combined withliquid from the first loop from step (b) to pass directly to the thirdloop.